Praise be to Allaah.
Obedience to a created being becomes shirk in situations such as obeying that person in matters where what is forbidden is deemed to be allowed, and vice versa, or when that person issues rules and regulations, or laws, that go against the laws of Allaah, and the follower believes that these laws are more complete and better than the laws of Allaah, or as good as His laws, or that the laws of Allaah are better but it is permissible to follow these man-made laws. The evidence for this is the aayah (interpretation of the meaning): “They (Jews and Christians) took their rabbis and their monks to be their Lords besides Allaah (by obeying then in things which they made lawful or unlawful according to their own desires without being ordered by Allaah)…” [al-Tawbah 9:31]. ‘Adiyy ibn Haatim said: “O Messenger of Allaah, we do not worship them.”[He was a Christian who became a Muslim - Translator]. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do they not allow you that which Allaah has forbidden and forbid what Allaah has allowed?” He said, “Yes.” He said, “This is a kind of worship.” The Christians’ obedience of their priests in sin and in accepting what they say about what is lawful or unlawful is a form of worshipping something other than Allaah, which is a kind of shirk akbar (major shirk), that contradicts Tawheed (true monotheism).
With regard to your question, if the person who obeys his parents in sin believes that it is in fact a sin, but he does what he does because of his own desires or because he is afraid of his parents’ punishment, but it is not the matter of being forced to do so, then he is a sinner who has gone against the words of the Prophet (peace and blessings of Allaah be upon him), There is no obedience to a created being if it involves disobedience of the Creator”(reported by Ahmad, 1041; a saheeh hadeeth) – but he is not considered to be guilty of shirk akbar. But if the son believes that what his parents say overrules what Allaah says about halaal and haraam, then he is guilty of shirk akbar. The Muslim has to strive against himself to bring his desires in line with what the Prophet (peace and blessings of Allaah be upon him) taught, and so that he gives precedence to obeying Allaah and His Messenger over obeying anyone else, and so that Allaah and His Messenger are more beloved to him than anyone else. The Prophet (peace and blessings of Allaah be upon him) said: “None of you truly believes until I am more beloved to him than his son, his father, and all the people.” (Reported by al-Bukhaari, 63). And Allaah is the Guide to the Straight Path.